Optics in Astronomy- Interferometry -
Oskar von der LüheKiepenheuer-Institut für Sonnenphysik
Freiburg, Germany
September 2002 Optics in Astronomy 2
Contents
• Fundamentals of interferometry
• Concepts of interferometry (contd.)
• Practical interferometry
• Concepts of interferometry (contd.)– Differential delay tracking
– Observables
– Sensitivity
• Practical interferometry– Todays Interferometers and Science cases
September 2002 Optics in Astronomy 4
Differential delay tracking
Goals of DDT:
• off-source phase referencing
• narrow-angle astrometry
Source of figures: PTI
September 2002 Optics in Astronomy 5
Measured Quantities and Observables
The measured quantity of interest is the correlated flux at wavelength and angle frequency ikik Bu
dujIuI ikFOV
ik 2expˆFourier component of sourceintensity:
dI
dujI
I
uIV
FOV
ikFOVik
ik
2exp
0ˆ
ˆComplex visibility:
September 2002 Optics in Astronomy 6
Observables I
Group Delay :Delay for which interference contrast is maximisedGroup delay depends on • baseline• instrumental fixed delays• source position• delay errors: optics, vibrations, atmosphere
High precision measurement of permit relative position measurements with 1mas accuracy over wide angles (many degrees) and with ~10µas accuracy over narrow angles (arcminutes): Mark III, PTI
Calibration with reference stars or optical truss anchored to earth crust: NPOI
Astrometry!
ik,0
ik,0
September 2002 Optics in Astronomy 7
Observables II
Visibility Amplitude :• maximum contrast in interferogram• visibility as function of delay depends on spatio-spectral
content of source and system throughput• essential for imaging with photometric fidelity• calibration through rapid switching between program and
reference sources with known visibility, monitoring of system parameters
Maps, Images!
ikV
ik
September 2002 Optics in Astronomy 8
Observables III
Referenced phase :• argument of complex visibility:• can be referenced to off-set calibrator source by differential
delay measurements• can be referenced to program source at different wavelength:
GI2T• essential for imaging• Raw visibility phases are no good observables due to
uncontrolled errors: • Visibility phase can be re-transformed by changing origin of
coordinate system without affecting the morphology of the reconstructed image
Maps, Images!
ikikik jV exp
jkikik ,0
September 2002 Optics in Astronomy 9
Observables IV
Closure phase :• triple product of complex visibility:
• good observable provided there are no baseline-dependent error sources
• insensitive to source position• fewer independent closure relations than baselines:
vs.
• essential for imaging if there are no referenced phases
Maps, Images!
ikl
liklik
liklikikl
,0,0,0
112
NN 12
NN
September 2002 Optics in Astronomy 11
Signal-to-noise ratio of a visibility measurement
KN
VN
NNNS
NSSNR
tel
red
dbp
p
21
S system Strehl, = 1 with mode stop, = 0 w/o mode stopNp detected number of source photonsNb detected number of background photonsNd detector noise, expressed as equivalent rms no. of photonsNred redundancy of baseline consideredNtel total number of telescopes in arrayV intrinsic visibility of sourceK number of incoherently averaged visibility measurements
September 2002 Optics in Astronomy 13
Year Scientist Remarks 1868 H. Fizeau Proposal to use masks to increase telescope resolution 1870 E. Stephan Marseille 80cm reflector with strip mask 1890 A. Michelson Diameters of Jovian satellites (Lick) 1921 A. Michelson, F. Pease Diameter measurement of Ori with 20 ft.
interferometer on Mt. Wilson Hooker telescope 1935 F. Pease Mt. Wilson 50 ft. interferometer (unsuccessful) 1956 R. H. Brown, R. Twiss Intensity Interferometer 1970 A. Labeyrie Stellar Speckle Interferometry 1973-1975 A. Labeyrie Interferometry w. independent telescopes (24cm) 1974 M. Johnson et al. Heterodyne interferometry at 10 µm 1985 A. Labeyrie Interferometry w. independent telescopes (150cm) 1988-1993 M. Shao et al. Production-line interferometry 1990 J. Baldwin et al. Phase-closure imaging of Ori surface 1995 J. Baldwin et al. Multiple telescopes imaging of Capella 2001 M. Shao et al. "First fringes" with Keck Imaging Interferometer Array 2001 A. Glindemann et al. "First fringes" with VLT Interferometer siderostats
Milestones in Optical InterferometryMilestones in Optical Interferometry
September 2002 Optics in Astronomy 14
Program (Nation) No. of. Baselines Max Baseline [m] Element Diameter[m]
Year of Operation
GI2T (F) 3 65 1.52 1985
ISI (USA)1 1 35 1.65 1988
COAST (GB) 6 100 0.40 1992
SUSI (AUS) 1 640 0.14 1992
IOTA (USA) 3 45 0.45 1993
NPOI (USA) 3 (6, 15)2 250 0.35 1995
PTI (USA) 1 110 0.45 1996
MIRA-I (JN) 1 4 0.20 1998
CHARA (USA) 10 350 1.00 2000
VLTI (EUR) 6 / 3 / 63 128 / 2004 8 / 1.8 2001
KIIA (USA) 1 / 6 / 153 75 / 1804 10 / 1.5 2001
Magellan (USA) 1 60 6.5 2005
LBT (USA/I/D)5 1 20 8 2005
Today‘s InterferometersToday‘s Interferometers
September 2002 Optics in Astronomy 23
Fundamental Stellar Parameters
Fringe contrast as function of apparent diameter
Distribution of apparent diameters for various spectral classes for stars seen from Paranal, Chile
September 2002 Optics in Astronomy 25
Stellar Surfaces
700 nm (WHT) 905 nm (COAST) 1290 nm (COAST)
Three maps of Ori (Betelgeuse) taken in Nov. 1997
pictures courtesy COAST
September 2002 Optics in Astronomy 28
Observations of Mizar with NPOI on May 1 - 4, 29, June 1, 1996
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